Calibration system for radio receivers



l Jan. 279, 1946.

A. A.` COLLINS v CALIBRATION SYSTEM FOR RADIO RECEI'VERS 2 Sheets-Sheet 2 Patented Jan. 29, 1946 CALIBRATION SYSTEM FOR RADI RECEIVERS Arthur A. Collins, Cedar Rapids, Iowa, assignor to Collins Radio Company, a corporation of Iowa Application October 12, 1944, Serial No. 558,412

(Cl. Z50-39) 8 Claims.

This invention relates to a calibration system, and more particularly to a system for Calibrating the frequency to which radio apparatus is tuned.

One feature of this invention is that it provides means for Calibrating or indicating the frequency to which radio apparatus, and more particularly receiving apparatus, is tuned; another feature of this invention is that the Calibrating and indicating iseiected with a high degree of accuracy even when the receiver is tuned to relatively high frequencies, as those used in aircraft communications; still another feature of this invention is that the calibrating or setting of the receiver to the desired frequency may be done very quickly and easily; and yet another feature of this invention is that the advantages may be obtained withoutl complicated mechanical arrangements or extreme and unusual precision and control of production and assembly of mechanical or electricalvparts. Other features and advantages of this invention will be apparent from the following specification and the drawings, in which:

Figure 1 is a block diagram illustrating the principles of my invention; and Figure 2 is a circuit diagram of simple radio'receiving aparatus embodying the system of Figure 1.

Under some circumstances, particularly in communication work, it is desirable to be able to tune receiving aparatus' to a predetermined frequency on which transmission is to be expected, even though at the time the tuning is done no transmission 'is takingplace and Calibrating and indicating means must be used to determine whether the receiver is tuned to the desired channel. It may also be desirable under some conditions to have Calibrating and indicating means of sul-A cient accuracy that the receiving apparatus may readily determine Which of two closely adjacent signals is the one it desires to receive; or to use the receiver as a frequency meter. This matter of providing a suiciently accurateindication of the frequency to whichv the receiver is tuned is not particularly diicult at low frequencies, as those in the broadcast band. The problem is considerably different, howeven in higher frequency bands, particularly where a receiver incorporates a band switching arrangement and the same main tuning dial must function in each of a plurality of different bands. Accurate Calibration by use of the main tuning dial under such circumstances requires the dial mechanism and other mechanical parts and the electrical'elements of the associated tuned circuits to be held within tolerances which arev impossible from' a `of the'frequency to which the receiver is tuned,

regardless of the band in which it may be operative. I accomplish this by providing a source of a plurality of Calibrating signals of equally spaced fixed frequencies which have been very accurate.

ly calibrated; selecting, by means of the main tuning oscillator and radio frequency tuning arrangements, one of these Calibrating signals closely adjacent the channel on which it is desired to set the receiver, the action of the main tuning oscillator beating this chosen Calibrating signal down to an intermediate frequency. In order to provide an accurate interpolation or exact determination oil the frequency I beat this intermediate frequency signal with a Calibrating oscillator or beat frequency oscillator variable over .a frequency range equal to that between calibrating signals and relatively accurately calibrated, preferably providing a tunable intermediate frequency amplifier and synchronizing variation o1' the calibrating oscillator with variation of this intermediate frequency amplifier. The result is that the dial or other indicating means associated with the main tuning oscillator need have its indicating means only calibrated with an approximate or reasonable degree of accuracy easily maintained as a production matter; and the more exact interpolation is provided by the relatively low frequency Calibrating oscillator working at or near the normal intermediate frequency of the receiver.v

In the particular embodiment of this invention illustrated in Figure 1, a receiving system oi' the superheterodyne type is shown as comprising a radio frequency ampli-ner portion A, a mixer or rst detector B, a main tuning oscillator C, an intermediate frequency amplifier D, a detector or .second mixer E, and an audio frequency amplif lier F. When the switches'G and H are in the dotted line positions, the receiver operates as a Conventional superheterodyne receiver, a signal picked' up by the antenna J undergoing prelimilier F and delivered to earphones or other transfV lating means K. The radio frequency amplifier A, mixer B, and main tuning oscillator Ciara ali; Y indicated as being variable, theirl variable tuned* circuits being ganged and synchronously varied; by any appropriate manually operablef'meananje; the main tuning dial L. During ordinary-reception the intermediate frequencyfr amplier D, al-V though indicated as variable, wouldbe' peaked aty a predetermined intermediate frequency sometimes referred to hereafter as the reference frefluency: Y g .c

c When' it is' desired accuratelyftol determine the frequency on which reception is taking p'1ace; or totunethe receiver precisely to a desired channel, the switchl G is thrown from'th'eposi-tion shown inY dotted lines' to* the` position shown in *fullv lines',.connecting the input ofthe receiver to a source of a plurality of Calibrating signals of equally spaced' fixed frequencies', thezblock representing thissource being here* identified as These-frequencies' preferablydifferby'a given frequency which isy a very 'smallfia-ctionof the frequencies which the set is designed to receive; and the given frequency ispreferably-terr-t'o-an-V iintegrali-power cycles; If'the receiveriis designed to operate in a 2.5'-to= 3.5 megacyclebandjor exjample; thegiven frequency would' preferably be 100 kilocycles. I'he source of' calibrated frequencies would beV sok constructed and arranged asy to` provide Calibrating signals at frequencies exactly 100 kilocycl'es" apart; at least, in the band between2-5 and bmegacycles'.

A The calibratingV arrangement also includes a beat` frequencyI or calibrating frequency oscilla- Y torrN adapted to be connected" to or disconnected from` the detector El by the4 switch H, When Calibrating or setting'isbeing donethe switch H wouldbe in theposition shown in1sol`1'd'1in'es; connecting the oscillator N to the; detector to cause it to beat with the intermediate frequency signal delivered from the intermediate frequency Vamplifier` D; The beat frequency or Calibrating oscillator is tunable over `a frequencyrangel equal oscillator. With my system of frequency calibration it is thisbeat frequency or calibrating oscillator dial which provides the accurate indication of the frequency to which the receiver is tuned, the main tuning dial L merely providing an approximate indication which need only be sufficientlyaccurateto determine which of the Calibrating signals spaced. 1100,l kilocycl'es:v is being passed through the system. Moreover, extremely accurate setting of thev calibrating oscillator N is easily and quickly effected by beating its output 'againstfthe intermediate frequency signal until zero beat is obtained, my system not depending on relative strength of signal, or the like.` Y v While my system is operable with a fixedly tuned intermediate frequency amplifier having fairly wide band pass characteristics, I prefer to use4 an intermediate frequency amplifier having its tuned circuits relatively sharply peaked, but tunable ory variable over the same frequency range as that'v covered by the Calibrating oscillator.` Inv thev particular casex assumed above; for example; thetunedj circuits of the intermediate frequencyA amplifier Dl would-I preiferably pass a bandonl-y 5# or 1'0'1kilocycles wide, andrwould be tunablev over theirange from` 0l` to 550 kilocycles. 'dkilocycles would bethe'normal intermediate frequency of' the set, used during reception., this frequency being the reference frequencyif from which'- departures are made for accurate calibration. Wliensuchdepartures-are made, of course; the l system would temporarily have a somewhat different intermediateI frequency depending-upon the setti-ng of the intermediate frequency ampliffier:V Y The tuned' circuits ofthis` ampliierD and of' the- Calibrating oscillator Ni are'- synchronized and gan-ged to be varied byl asingle tun-ing contro-li, indicated in the drawings as; the dial 0)- 'I'he-ease, simplicityandl accuracy of* this calibrati'on* system willil be-readilyapparent from an explanation of the manner in which. itis used', and

` a representative example will be described: Let

uswassume thatwed'esireftozsetzour receiver to the channel .onwhich a-Vknown transmitterwijll operatein: the.l near future, which may befassumed to be 3,126 kilocycl'es.4 Remembering thatwehave xed accuratel Calibrating signals yavailablefrom the sourceY aty 3,000 kc.,. 3 ,00 kc., 33200i ker,V etc.,

willbe seenJthat the desired frequency is closest to the.Y 3;]00 kilocyclef Calibrating signal, and." that the difficult partl of' the calibration would, under normals circumstances, be theaccurate determinatothe difference between Calibrating frequencies delivered by the source M, and including the intermedia-te frequency. The intermediate freqnen-cyis preferably sov chosen thatI the frequency over which the Calibrating oscillator is tunable is` asubstantialkr fraction of the intermediate frequency'.- For example, in connection withV the frequencies assumed` inthe precedingparagraph', the intermediate'- frequency might well be 500 kilocycles, andthe oscillator N tunable overa range of from 450 kilo-cycles to 550- kilocycles; The; provision o-f this beat frequency or calibrating oscillator, operating at a frequency where relatively accurate calibration of its dial is easily effected, is of pri-me importance in the practice of' my invention. It is a relatively simpleeproduction' matter to build an oscillator operating throughtheabove-mentioned frequency rangeA at a frequency around 400 toy 5610` kilo'ey'cles with considerable stability. and with quite accurate correlationbetweenv theV scale graduations on its tuning dia-1 andthe frequency delivered by the tionlofrapointr 261kil`ocycl'es above this Calibrating signal, Ina commercial receiver embodying my invention the Calibrating dial; l1 would normally be: releasablylocked ata central positi'om thefzero or reference position, with the circuits, ofthe intermediate frequency amplifier D?. and` calibrating oscillator N: tuned to u kilocycles, Thetrstr step would;vr be' to unlock the dial 0 (as by applying suicientsforce to. movel a spring-pressed. ball'out of a cooperatingrdetentr and rotate it what mayh be, termed a; positiver direction until the graduations of" this; dial showr that the. frequency is 26 kilocycles above. the reference frequency, so

v that: intermediate frequencyfamplier D: and

calibratingfoscillatorl would then be tunedto 526 kilocycles.; Thamaintuning dial L. would; then be rotated to. a positionr slightly above the 3,100

kilocyclee marking orv graduation, and. this: dial rotated. slowly-back and.- forth in the neighborhood off this: point-,until a, zero beatwas secured' between the output of the Calibrating oscillation N and the; particlil-larY fixed: calilnzating,y frequency from the source M which reached the detector E. The tuned circuits of the RF amplifier A and mixer B would be sharp enough to provide sufficient elimination, at or near the 3,100 kilocycle position of the tuning dial L, of all calibrating signals other than the desired 3,100 kilocycle signal. When this signal zero beats with the 526 kilocycle calibrating oscillator output, the main tuning oscillator C will be accurately set at a frequency differing by 526 kilocycles from the 3,100 kilocycle calibrating signal, as for example at 3,626 kilocycles. The main tuning dialL would then be left in this position, and the calibrated dia1 l! rotated back to the reference position, again setting the tuned circuits of the intermediate frequency amplifier D at 500 kilocycles. If the switches G and H are then thrown to their dotted line positions, the calibrating oscillator N will then be removed from its connection to the system and the input of the receiver will again be connected to the antenna J. Signals reaching this antenna and delivered to the mixer B will then beat against a main tuning oscillator frequency of 3,626 kilocycles; and, since the intermediate frequency amplifier lis now peaked to an intermediate fre-l quency of 500 kilocycles, the received signal, the channel to which the receiver is accurately tuned, is 3.126 kilocycles.

While the above representative example is believed suiiicient for purposes of understanding of myYinVent-ion, it will be understood that the arrangement may be used not only Afor setting the receiver in advance to a desired channel, but also for using the receiver as an accurate wave meter to determine the frequency of any signal within the band range of the receiver. If a given station is being heard, its approximate frequency may be read from the graduations on the main tuning dial L, an exact determination of its frequency being achieved by throwing the switches to the calibrating position and tuning the dial until zero beat is secured between the calibrating oscillator and the particular calibrating signal selected. by the tuned circuits in the radio frequency portion of the receiver. In any case, the correlation between the graduations on the main tuning dial L and the frequency to which the receiver is tuned need only be approximate, the accurate interpolation or determination being provided by the graduations on the dial 0. In the example assumed above, for example, the main tuning dial L might have read about 3,115 kilocycles, but the fact that zero beat was effected when the calibrating dial. 0 was at plus 26 kilocycles indicated that the true frequency to which the receiver was tuned was 3,126 kilocycles. Moreover, while the system has been described above, and will be shown in circuit form in Figure 2, in connection with a receiver covering only a single band, it will be understood that the system is equally operable and is' particularly advantageous in connection with multi-band receivers. The fixed reactance elements in the tuned tank circuits in the main tuning oscillator C.and the radio frequency portions of the receiver may be switched to. provide the variation between different bands, even though this accentuates the discrepancies between the indications on the main tuning dial L and the true frequency in any particular case. 'Ihe reactance elements of the tuned circuits controlling the frequency of ,f

calibration desired in any of the bandsthrough which the receiver is tunable.

"Figure 2 is Aa simplified circuit diagram of a radio -receiver embodying my invention, the receiver being shown as covering only a single band, although it will be understood that in practice the xed reactancey elements of the tuned tank circuits would be switched to provide coverage of a plurality of bands. The radio frequency arnplifier portion A, adapted to be selectively connected either to the antenna or to the calibrating frequency source by operation of th'e switch G, is here shown as including a tube IB, which may beof the 12SG7 type. The input is through a radio frequency transformer having its secondary coil I I permeability tuned to provide the variable reactance element of a tuned tank circuit including the fixed condenser I2. While the tuning is here sh'own as of the variable inductance type, it will be understood that the invention is equally applicable to tank circuits wherein the inductance is the xed reactance and a variable condenser is used for tuning. AWhile gauging or synchronizing of the variable elements in the tuned circuits' throughout the diagram has notbeen specifically indicated th'ereon, as connecting lines would unduly complicate the circuit diagram, it will be understood that all variable coils are ganged with at least some of the other coils. As was described in connection with the block diagram, the tuned circuits of the radio frequency amplifier, mixer and main tuning oscillator all have their variable elements ganged for operation by themain tuning dial; and the tuned circuits of the intermediate frequency amplier and the calibrating oscillator have their variable elements ganged for synchronous operation by the calibrating dial.

The output of the radio frequency section of the receiver (which in practice would normally comprise several tubesin cascade, rather than the single tube shown for simplicity of illustration), is coupled to th'e input of the mixer or first detector tube I3, which may be of the 12SA7 type. This coupling is here shown Aas effected by a coupling condenser I4 and a pair of coupled tuned coils I 5 and I6, these coils having the fixed condensers I'I and I8 in shunt therewith.

Connection to the injection grid of the mixer tube I3 is a lead from th'e output of the main tuning oscillator of the reeciver, here shown as including the tube I9, which may be of the 12SJ 7 type. The frequency o-f this oscillator is controlled by a tank circuit including the variable inductance 20 and the fixed condenser 2|. In

the particular receiver here being described as a Y representative example, the tuned circuits in th'ey radio frequency section in the input of the mixer might cover the range of 2.5 to 3.5`megacycles; and the tuned circuit of the main oscillator might be arranged to cover the range of 3 to 4 megacycles, the Variation throughout these ranges being coordinated, as heretofore mentioned, so th'at the beat note output of the mixer tube I3 is 500 kilocycles.

This intermediate frequency mixer output is delivered to the intermediate frequency amplifier section of the receiver, this section being here shown as'comprising the single tube 22, which may be of the 12SG7 type, although' in practice a plurality of such tubes and associated circuits would normally be arranged in cascade in this section of the receiver. Both the'input and output of the intermediate frequency amplifying tube 22 is eifected'through doubly tuned coupling arrangements. The input coupling arrangement is here shown as including the couplingconquency signal is delivered tov the anodes of this tube, the. anodes and cathodes'being. respectively tied together in this case. so that vthe tube operates as a singlet rectifier or detector.y The output of this. tube 33 is delivered to an audio; frequency amplifier section which would in practice generally comprisev several' tubes in cascade, but. which is here shown as comprising the single tube 34, which may be of the. 128.17 type. Th'e vanfipliiled audio output is delivered' to any appropriatey translating dem'ce, as thev earphones K.

The source .of calibrating signals is the circuit section at the lower left-hand corner of the figure, here. shown as including the tubev 35', which' may be of the. 12SJ7 type. This tube is arranged to operate as an oscillator.. at a fixed frequency, as for example.V 100' kilocycles, determined by the flxedly tuned tank circuit including the coil 36 and the condenser 31. Coupling to a signal grid is.. effected through a 100 kilocycle crystal 38.

The crystal insures oscillation at the. desired fundamental frequency of 100 kilocycles; and proper adjustment of th'e circuit components in known manner `provides an .output of the oscillator 35 which is 1rich -in harmonics of this fundamental, the th to 35th harmonics being used for calibrating purposes in the circuit here being particularly described. In order to prevent undesired interference, from the Calibrating signals during reception, .the tube 35 may be rendered inoperative by operationof th'e screen grid voltage supply switch 39. Plate. supply voltage isv delivered to the oscillator 35 through a blocking resistor 40 and radio frequency choke 4i, isolating the radio frequency output from direct connection with the remainder of the circuit except through the couplingA condenser 42 and switch G to the input of the radio frequency amplifying section.

The Calibrating' or beat frequency oscillator section N is here. shown as comprising a tube 43, which may ybe of the 12SJ7 type, the frequency generated by this oscillator being determined by a tunable. tank circuit `including the variable inductance 44 and the fixed condenser 45 in shunt therewith. While the switch H'was shown on the block diagram of Figure 1 as direct connected between the beat frequency oscillator and Y the detector, switching is in practice preferably accomplished electronically by using the switch H to connect .the plate of the tube 43 to, or disconnect it. from, the source of plate Voltage. In the particular receiver being described as representative, the coils. 24, 25', 29', 30 and 44 would be ganged for synchronous operation, the reactance elements of the tank circuits thus tuned being so chosen .that the intermediate frequency amplifier tuned circuits and the oscillator frequency would synchronously cover a range of from 450 to 5,50 kilocycles. rIhe center of this range, the 500 kilocycle frequency, would be used' as the reference or normal intermediate frequency, with. the circuits being tuned to this irequencyduring normal reception Vand displaced therefrom only for calibrating purposes.

, vTo cover the frequency ranges specified above, the coils ll, .I 5. and I6 might-be variable from 5.7 to 11.3, microhenrys, and shunted by-360 micromicrofarad condensers I2, l1 and I8; the main tuning oscillator tank circuit inductance 20 might bevariable from 5.1 to 9.2 microhenrys, and be shunted by a 310 mmf; condenser 2|; the intermediate frequency tuned coils 24,25, 29 land v3.0 might be variable from 163 to 240V microhenrys, and be shunted by 520 mmf. condensers; the Vcalibrating oscillator tank circuit coil l44 might be variable from 46to 68 microhenrysa-nd be' shunted by a 1,850 mmf. condenser 45; and the xedlyl tuned tank circuitof the harmonic generator Vor Calibrating signal generating oscillator might have a tank clcuitwth its coil 36 of 1,000 microhenrys and its condenser 31 of 2,500 mmf. capacity.

Other circuit .elements associated with the varif ous tubes would be of conventional value for the particular applications and functions of tubes of the types given in each case. For example, in the radio frequency amplier'section, the cathode bias resistor and screen grid resistor might be of and 100,000 ohms, respectively; and the bypassing condensers in each case might have capacities'of ,0.1 mf. In the mixer, the input coupllflgy condenser and output coupling condensers i4 and 23 might have capacities of 4 and 10 mmf., respectively; the injection grid might be by-passed to ground through a 20,000 ohm resistor; and the cathode biasing and screen 'grid resistors might be of 220 and 47,000 ohms, respectively, being ley-passed by .0l'mf. condensers in each case. In the intermediate frequency amplifying section, the cathode' and screen gridV resistors might be of 100 and 22,000 ohms, respectively, again being by-passed by .0l mf. condensers in each case. In the detector section, a cathode biasing resistor of 470,000 ohms might be used, 'by-passed by a 250 mmf. condenser; and the condensers coupling the anodes to the Calibrating oscillator, and the cath-l odes to the audio frequency amplienmight have capacities of 20 mmf. and .006 mf., respectively. The cathode biasing and control grid resistors in the audio amplifying section might be of 500. and

47,000 ohms, respectively, with the cathode resistor Iley-passed by a 4 mflcondenser. In the oscillators, the grid of the tube 35 connected to thev switch 39 might have a 47,000 Yohm resistor in seriesV therewith, by-passed by a .01 mf. condenser, with a 470,000 ohm resistor shunting the crystal 38; in connection with the main tuning oscillator tube I9, a coupling condenser of 1'00 mmf. and a resistance to ground of 150,000 ohms might be used, with the screen grid having a resistor of 68,000 ohms by-passed by a .01 mf. condenser; and in the Calibrating oscillator, the grid leak resistorV and condenser might have values of 150,000 ohmsl and 100 mmf., respectively, with the screen vgrid having. associatedV therewithV a 68,000 ohm resistor ley-passed by a .0.1 mf. condenser.

While I have shown and described certain embodiments Aof my invention, it is to be understood that it is capable of many modifications. Ghanges, therefore, in the construction and arrangement may be made without departing from the spirit and scope of vthe invention as disclosed in the appended claims..

I claim: Y

1. Radio apparatus ofthe character described, including: a source. of al Calibrating signal of'iixed frequency; a tuning oscillator adapted `to beat said Signal dOWn toy an intermediate frequency;

a calibrating oscillator variable over a relatively,

limited frequency'rangeincluding said intermedie eaeae.

ate frequency and adaptedio .beat with said quency range equalfto-thatjbetween Calibrating lntermedlate frequency signal; andl indicating signals and including said'yintermediate fremeans for indicating the-'variation of the frefluency; a calibrating Oscillator variable overa fluency of the Calibrating oscillator from a predefrequency frange equal tothat betweenfcalibratmg termlned reference frequency. l 5 Signals and adapted t0 beat With the amplified in- 2. Radio apparatus of the character described, termediate frequency signal; means for synchroincluding: a source of a plurality of Calibrating nOuSly Varying the intermediate' frequencytamsignals of equally spaced fixed frequencies; a plier and Calibrating oscillator; and indicating tuning oscillator adapted to beat a selected one means fOr indicating the Variation cf the freof said signals down to an intermediate frequency; l0 quency cf the Calibrating Oscillator from a Drea Calibrating oscillator variable over a frequency determined reference frequency.

range equa1 to that bei-,Ween Calibrating signals 6. Radio receiving apparatus of the character and including Said intermediate frequency and described, including: variable radio frequency amadapted to beat with Said intermediate frequency plifying means; a variable rst mixer connected Signal; and indicating means for inriicnting che thereto; a variable tuning oscillator connected to variation of the frequency of the Calibrating oscil- Said mixer; means fcl synchronously Varying the lator from a predetermined reference frequency. amplifier mixer and Oscillator? a Scul'cc ci a Plu- 3; Radio receiving apparatus of the character faulty 0f Calibrating Signals 0f Vequally spaced described including: radio frequency amplifying fixed frequencies, the spacing between frequencies means; mixing means, at least one of these means being only a small fraction of such frequencies; including variable tuning means;ia variable tunmeans for Supplying said Signals to the amplifier ing Oscillator; means for synchronously Varying and mixer, whereby the oscillator beats a sethe oscillator and variable means; a source-of a iecteu one 0f Said signals down t0 un intermedi' plurality of Calibrating signals of equally spaced atc frequency; indicating means aSSOciatcd With fixed frequencies; means for supplying said sig- Scl/1d SYnchrOnOuSly Varying means for alJDrcX- nais to the amplifier and mixero whereby the mately indicating the frequency to which the reoscillator beats a selected one of said signals down cclvlug apparatus is turned? an intermediate freto an intermediate frequency; a Calibrating osciluuency umpliiicl llaVlng tuning means Variable later variable over a frequency range equal tc that over a frequency range equal to that between Calbetween Calibrating signals and including saluv ibrcting Signals and including said intermediate intermedire frequency and adapted to bear with frequency; a Calibrating oscillator variable over said intermediate frequency signal; and lndicata frequency range equal to that between caling means for indicating the Variation of the ibla'tillg Signals and adapted to beat with the frequency of crie Calibrating Oscillator from a amplified intermediate frequency signal; means predetermined reference frequencyfor synchronously varying the intermediate fre- 4. Radio receiving apparatus of the CharaCter lucncy amplllier and Calibrating Oscillator; esecdescribed, including: radio frequency amplifying cud mixer connected t0 the intermediate fremeans; mixing means, at least one of these means queucy amplllicl and Calibrating Oscillator; and

including variable tuning means; a variable tunindicating means for indicating the Variation of ing csemater; means for synchronously Varying 40 the frequency of the Calibrating oscillator from v the oscillator and variable means; a source of a a predetermined reference frequencyvfrequency amplifier havingtuning means variplurality of Calibrating signals of equally spaced '7- Radio apparatus 0f the character described, fixed frequencies; means for supplying said sigincluding: a 50u-icc 0f a plurality 0f Calibrating nals to the amplifier and mixer, whereby the os- Signals 0i equally spaced i'lXed frequencies differ- Cillator beats a selected one of said signals down ing by a giVen frequency 0f ten-tO-an-integralto an intermediate frequency; an intermediate PcWcI cycles; a tuning Oscillatcr adapted t0 beat a. selected one of said signals down to an interable over a frequency range equal 'to between mediate frequenCy; a Calibrating Oscillator VaI`- Calibrating signals and including said intermediable 0Ver a frequency range equal to that beate frequency; a, Calibrating oscillator varlble tween cahbrating usuels and including said inover a frequency range equal to that between Caltermediate frequency and adapted to beat with lbratlng signals and adapted to beat with the ,Said intermediate frequency signal; and indicatamplied intermediate frequency Signal; means lng means for indicating the variation of the frefor synchronously varying the intermediate frefluency 0f the Calibrating Oscillatcr from a Drequency amplifier and Calibrating oscillator; and determined reference frequency.

indicating means for indicating the variation of 8 Radio receiving apparatus of the character the frequency of the Calibrating oscillator from a described, including: variable radio frequency predetermined reference frequency. amplifying means; a variable rst mixer con- 5, Radio receiving apparatus 0f the character nected thereto; a variable tuning oscillator Condescribed, including: radio frequency amplifying nected to said mixer; means for synchronously means; mixing means, at least one of these means Varying the amplifier. IniXcr, and oscillator; a including variable tuning means; a variable tunsource of a plurality 0f Calibrating signals of ing oscillator; means for synchronously varying equally spaced fixed frequencies, the spacing bethe oscillator and variable means; a source of a tween frequencies being only a small fraction of plurality of Calibrating signals of equally spaced such frequencies and differing by a given frexed frequencies, the spacing between frequencies quency 0f ten-to-au-integral-DOWer Cycles; means being only a small fraction of such frequencies; for Supplying Said signals to the amplifier and means for supplying said signals to the amplifier mixer, whereby the oscillator beats a selected one and mixer, whereby the oscillator beats a selected of said signals down to the intermediate freone of said signals down to an intermediate frequency; indicating means associated with said quency; indicating means associated with said synchronously varying means for approximately synchronously varying means for approximately indicating the frequency to which the receiving indicating the frequency to which the receiving apparatus is tuned; an intermediate frequency apparatus is tuned; an intermediate frequency amplifier having tuning means variable over a amplifier having tuning means variable overa frefrequency range equal to that between calibrating signals andv including saidfinigeimediate fre nii-bier conne'td `to the' intermediateV requencif quency; a Calibrating bseillater variable v(weit `a inplifierandealilbating'seillatom -afndindica;t frequency range equal 'to' that between calibrat mg means -SOrmdmating the vriaeion'of the fre; ine signalisatie-adapted togbeat'wth `the amplied quency 01 the Calibrating Aeseilmer .from 4'alpineinte'rmediate "frequency signal; lmeans for `syn- -5 determined reference frequency. chrono'sly varying Vthe intermediate frequency Y A.C0`LLINS. amplifier and calibrating' oscillator; 'a secondV v e Y. e 

